In Reply to: A good question merits a good answer... posted by J.D. on October 24, 1999 at 19:23:34:
: V-6s offer great packaging for today's "cab-forward" designs. They are especially well suited for front wheel drive cars. Most importantly, V-6s are often derived fron V-8s. American manufacturers often do this.
Very few V6s are derived from V8s anymore, at least for automotive applications. The optimum angle for a V8 isn't the same as for a V6, which means when you make a V6 out of a V8, you either get a rough V6 or need to put in a balance shaft. Same thing works the other way--witness the balance-shaft equipped, V6 derived V8 in the most recent SHO Taurus.
: A flat six, as used by Porsche, offers a lower center of gravity. It is essentially a V-6 with 180 degrees between the cylinder banks. They offer great smoothness. This engine layout also allows for lower bodywork, although the bodywork must also be wider to accomdate the engine -- a trade off. Flat sixes are not suited very well for front-wheel drive cars (transversely mounted engines). They are just too darn wide.
If I'm not mistaken, one of the primary reasons Porsche used a flat design was because it was better suited to air cooling (can mount the fan on top to blow through the cylinders, without worrying about the V). Same with VW, obviously.
: Inline/Straight Six:
: The I-6 is one of the "smoothest" engine designs. The layout of the engine naturally counteracts all vibration. For some reason though, the 4.0 L straight six used in the Jeep Grand Cherokee is far from smooth. Leave it to Daimler-Chrysler to screw up an essentially smooth engine.
Don't blame Stuttgart or Auburn Hills--blame Kenosha. The Jeep 4.0L dates from way back in the AMC days. Although it's been reworked several times since, the 4.0L was designed as a torquey truck engine, and its architecture reflects that. Service in a $30K+ luxury vehicle was never in its original design requirements.
: In addition to smoothness, I-6s, as a rule of thumb, are often torqueir than an equal sized flat- or V-6. Additionally, they allow for easy packaging in the long, narrow hood of cars like the Z3.
: BMW goes so far as to counter-wieght the camshaft lobes! They want to make sure that the essentially smooth I-6 stays smooth!
: Hopefully this answers your question.
: J.D. (Jonathan)